The present invention relates to a high sensitive multi-wavelength spectral analyzer which is capable of spectral analysis of extremely weak radiation over a spectral range of from the visible region to the infrared region, for example, bioluminescence, chemiluminescence, extremely weak fluorescence caused by excitation light, Raman scattered light, etc.
Recently, attention has been attracted to extremely weak radiation such as bioluminescence, chemiluminescence, extremely weak fluorescence caused by excitation light, Raman scattered light, etc. For spectral analysis of such extremely weak radiation, it is desirable to carry out measurement with minimal loss of light. For this purpose, it is preferable to use an optical system of high luminosity which enables a large exit solid angle to be obtained so as to increase the utilization of light flux (i.e., throughput), and arrange a detection system having the simultaneous photometry (i.e., multiplex) advantage that a multiplicity of wavelengths are measured at a time. More specifically, it is necessary to employ optical systems of high luminosity for both collimator and condenser systems and use a high sensitive multi-wavelength spectral analyzer arranged by adopting the design idea of a polychromator that is capable of multi-wavelength spectral analysis by simultaneously measuring a multiplicity of wavelengths in an observation wavelength region without performing wavelength scanning. However, there has heretofore been no spectral analyzer that meets the requirements. The conventional spectroscope employing a diffraction grating is basically a monochromator, and therefore it needs an entrance slit and an exit slit indispensably and hence necessitates wavelength scanning. During the wavelength scanning, rays of light other than the light that is taken out through the exit slit are discarded. Thus, this type of conventional spectroscope has no multiplex advantage.
In addition, the prior art is not designed so that a one- or two-dimensional light distribution detector for detecting a spectral distribution is disposed at the exit plane. Accordingly, when the exit slit is merely replaced with a detector, the wavelength region within which simultaneous photometry can be effected is narrow, and there are also problems concerning focal plane, aberration and so forth. Further, since both the collimator and condenser systems employ reflecting mirrors in an off-axis state, the spectral line image is curved owing to the off-axis arrangement and aberration, so that the f-number cannot be made very small. Thus, there has been a limitation on the luminosity of the spectroscope; no spectroscope of f3 or less has been put to practical use.
A spectroscope with f1 has recently been developed which employs a parabolic mirror as a reflecting mirror used in the prior art. However, since the converging mirror (parabolic mirror) is employed in an off-axis state, the spectral line image is curved, so that it is difficult to obtain an accurate spectral distribution with a one- or two-dimensional light distribution detector. In addition, since the detecting surface of the detector must be cooled in order to realize high-sensitivity measurement, a relatively thick vacuum chamber for heat insulation needs to be disposed in front of the photoelectric surface of the one- or two-dimensional photodetector, as a means for preventing moisture condensation on the detecting surface, so that it is necessary to increase the focal length of the converging parabolic mirror.
Further, a polychromator that employs a concave diffraction grating and a detector array has been put on sale recently. However, since there are limitations on the diameter and curvature radius of the concave diffraction grating, the light-gathering capability is limited, so that no adequate luminosity can be obtained.
In the meantime, it has been proposed to utilize a static interference spectroscopy employing a triangle common path interferometer, a quadrangle common path interferometer, a birefringent polarization interferometer, etc. for spectral analysis of extremely weak radiation. However, examination of this prior art presents a question about the statement that these interferometers have higher luminosity than that of conventional spectroscopes because there is no limitation on the area of the surface of a luminous sample. In addition, although these interferometers have an optical system which provides a higher luminosity than that of the conventional spectroscopes, the high luminosity is not characteristic of the interferometer, but it is owing to the use of lenses of higher luminosity in places of mirrors. Since the width of the detector array is smaller than that of the diffraction grating in the present state of art, the size of these widths determines the resolving power of each individual spectroscopic system, and therefore the resolving power of the static interference spectroscopy is inferior to that of the spectroscopes. With regard to energy, in the spectroscopes if the entrance slit width is enlarged, the resolving power lowers, but energy can be increased instead, whereas, in the static interference spectroscopy the contract deteriorates, resulting in no advantageous effect.
Thus, with the conventional spectroscopes or spectroscopic methods, it has heretofore been difficult to effect spectral analysis of extremely weak radiation such as bioluminescence, chemiluminescence, extremely weak fluorescence caused by excitation light, Raman scattered light, etc., particularly difficult to obtain simultaneously a spectral distribution of a multiplicity of wavelengths.